This invention relates to a process and apparatus for the fractionation of a gaseous mixture containing at least three components, said gaseous mixture being, for example, natural gas or the like.
DAS [German Published Application] No. 2,122,707 discloses a process wherein a gaseous mixture is fractionated in a first rectifying column into a head product largely freed of at least one higher-boiling component and into a bottoms product largely freed of at least one lower-boiling component. In this process, natural gas containing, in addition to methane, considerable amounts of nitrogen as well as carbon dioxide and higher hydrocarbons is initially fractionated in a first rectifying column into two fractions, the head product consisting essentially of nitrogen and methane, and the bottoms product containing not only a small amount of nitrogen, but also in addition to the main component methane, the entire amount of carbon dioxide contained in the natural gas, along with all heavier hydrocarbons. The bottoms product, a mixture of alkanes, is discharged into a pipeline after heating to ambient temperature, in the form of natural gas of low nitrogen content.
It is frequently desirable, however, to separate from a gaseous mixture several components having differing boiling ranges. For example, there is often interest in separating ethane and heavier hydrocarbons from natural gas in order to convert these components into more valuable products, such as ethylene, for example. The separation of a heavy fraction is also important where there is a demand for liquid gas. In addition, in the case of natural gases having a high nitrogen proportion, it is necessary in most instances to separate most of the nitrogen in order to obtain a particular caloric value for the end user.
To separate ethane and heavier hydrocarbons, as well as nitrogen, from natural gas, separate processes have heretofore been utilized; however, this is unsatisfactory, since the natural gas must be cooled down twice from the ambient temperature to the required low temperature levels.